As a form of an underpants-type disposable diaper, there is known an underpants-type disposable diaper including: a cylindrical outer body that is formed by joining a ventral side outer body and a dorsal side outer body at both sides; and an inner body that has a front part joined to a width direction central area of the ventral side outer body and a back part joined to a width direction central area of the dorsal side outer body and passes through the crotch portion of a wearer, the ventral side outer body and the dorsal side outer body being separated without being continued (for example, refer to Patent Document 1). In such an outer halved type, the ventral side outer body and the dorsal side outer body are not continued but separated at the crotch side, which provides an advantage that there is no need to punch leg openings for passage of the wearer's legs, or if there is a need, only small-area leg openings is enough. That is, as cut pieces (hereinafter, also called trims) are discarded, the material loss (hereinafter, also called trim loss) can be suppressed.
As a method for manufacturing the outer halved-type diapers, in general, the ventral side outer body and the dorsal side outer body are separately fabricated from belt-like continuous sheet materials, and the form illustrated in FIG. 1 of Patent Document 1 is also categorized as this method. In this case, it is necessary to provide an assembly line for the ventral side outer body and an assembly line for the dorsal side outer body in parallel to convey the outer bodies in parallel, which makes the production facilities larger and more complicated. This problem can be solved by supplying one belt-like continuous sheet material in a MD direction (mechanical direction or conveyance direction. The lateral direction orthogonal to this direction is called a CD direction) and disposing resilient and elastic members on the sheet material, and folding back the sheet material to cover the resilient and elastic members, then cutting the sheet material continuously in the direction parallel to the MD direction to divide the sheet material into a ventral side elastic belt and a dorsal side elastic belt, and then conveying the belts separated from each other in parallel in a width direction as described in paragraph 0037, Patent Document 1. In this case, however, at the step of folding back the sheet material, the wide sheet material is folded into two. This is not only making the folding facility (sailor) larger but causing a problem that it is difficult to fold the sheet material neatly without wrinkles. In addition, the edge of the waist of either the ventral side outer body or the dorsal side outer body may not make a fold in the sheet material (excellent in texture without turn-up of the sheet material) but may deteriorate in texture and appearance. Additionally, as described in Patent Document 1, when the wide sheet material is folded into two, the edges of the sheet material are likely to be misaligned, and the misalignment remains at the edge of the waist of the diaper to contribute to deterioration in texture and appearance.